It also could be a fungal disease. The link below is to a list of cannabis diseases that include a disease that causes yellow or brown spots on leaves. I'd like to mention to you that I have had these on various plants before and it didn't really do anything that I noticed but it can decrease yield if that's what it is. I too use the tap water where I live, and it has a ph of 6.5. I let mine sit out at least 24 hours also, and haven't had a problem with it. I do also flush after a month of growth and then again at the end.